Apparatus and process for working up a hydrogen- and methane-comprising stream

ABSTRACT

The invention relates to an apparatus ( 100 ) for working up a hydrogen- and methane-comprising stream ( 1.1 ), which comprises the following components:
         (i) at least one heat exchanger (KS 1 ) for cooling a stream ( 1.1 ) to be worked up;   (ii) at least one separation unit (A, A 1 , A 2 , A 2 ′) for purifying the stream ( 3 ) to be worked up to give a stream ( 5 ) rich in hydrogen and methane;   (iii) at least one cooling unit (KS 2 ) for cooling the stream ( 5 ) rich in hydrogen and methane; and   (iv) at least one cryogenic gas separation unit (KS 3 ) for separating the stream ( 6 ) rich in hydrogen and methane into at least one hydrogen-rich stream ( 7 ) and at least one methane-rich stream ( 8, 9 ).       

     The invention further relates to a process for working up a stream of material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit, under 35 U.S.C. §119(e), to U.S.Provisional Application No. 61/579,663, filed Dec. 23, 2011, which isincorporated herein by reference.

BACKGROUND

The invention relates to an apparatus for working up a hydrogen- andmethane-comprising stream, wherein the apparatus comprises at least oneheat exchanger, at least one separation unit, at least one cooling unitand at least one cryogenic gas separation unit. The invention furtherrelates to a process for working up a stream of material and the use ofthe apparatus or of the process for working up a stream extracted from ahydrodealkylation plant.

Aromatic hydrocarbons, in particular benzene, toluene, xylene andethylbenzene (also referred to as BTXE fraction), are among theindustrially most important bulk products in the chemical industry andare raw materials for plastics and other bulk chemicals.

Aromatic hydrocarbons are present, for example, in crude oil and apredominant part of the industrially important compounds are synthesizedby petrochemical processes such as steam cracking. Here, long-chainhydrocarbons are cracked in the presence of steam at residence times inthe millisecond range and temperatures in the range from 800 to 850° C.The objective here is to obtain short-chain alkenes, such as ethylene orpropylene. Aromatic hydrocarbons are firstly obtained as by-products andseparated by means of complicated separation processes into theindividual components such as benzene or toluene.

To cover the large demand for benzene, transformation processes whichallow alkyl-substituted aromatic hydrocarbons to be dealkylated, inparticular to benzene or toluene, have been developed. This makesflexible production matched to market conditions of aromatichydrocarbons possible.

An important transformation process is, apart from isomerization,hydrodealkylation. Here, an aromatic hydrocarbon such as toluene isconverted in the presence of hydrogen into a simpler aromatichydrocarbon such as benzene. This chemical process is described, forexample, in WO 2007/051851 and is generally carried out at hightemperatures, under high pressure or in the presence of a catalyst.

In hydrodealkylation, it is necessary to use a large excess of hydrogen,which means that hydrogen recycling and in particular separation ofhydrogen from the resulting product gas mixture have to meet demandingrequirements. One possibility is cryogenic separation, in whichcondensable impurities are separated off from the product gas mixture invarious cooling stages in a cryogenic gas separation unit. DE 20 55 507A1 discloses a process for purifying a feed gas composed of crudehydrogen, in which a crude hydrogen feed gas comprising condensableimpurities is subjected to stepwise cooling. For this purpose, the feedis passed through various cooling stages, the condensate is separatedoff from the feed after passage through each cooling stage, eachcondensate is depressurized and then passed in a recycle stream througheach preceding cooling stage for autogenous cooling. To maintain thecorrect heat balance of the process, the heat released in the lastcooling stage is given off to the outside, as a result of whichtemperatures down to −165° C. can be achieved. This cryogenicpurification plant enables hydrogen having a purity of more than 90% tobe obtained.

U.S. Pat. No. 3,371,126 describes a process for producing benzene andheating gas, in which, after a dealkylation unit, a stream rich inhydrogen and methane is purified in a cryogenic purification zone. Here,the stream coming from the dealkylation unit is firstly fractionated togive a benzene-rich fraction and a fraction rich in hydrogen andmethane. The fraction rich in hydrogen and methane comprises, forexample, 55.6 mol % of hydrogen, about 40.7 mol % of methane and about 4mol % of ethane. After the low-temperature fractionation, ahydrogen-rich stream comprising more than 80% by volume of hydrogen isprovided.

Known plants for the work-up and reuse of hydrogen use complex andcorrosion-sensitive components which have to meet highly demandingrequirements in respect of their efficiency, stability and safety.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows an illustrative embodiment of the invention as a processflow diagram.

DESCRIPTION

It is an object of the present invention to provide an apparatus and aprocess in order to simplify the work-up of product gas mixtures andprovide an inexpensive alternative which gives a high purity of theworked up hydrogen and meets high safety standards.

The object is achieved by an apparatus for working up a hydrogen- andmethane-comprising stream, which comprises the following components:

-   (i) at least one heat exchanger for cooling a stream to be worked    up;-   (ii) at least one separation unit for purifying the stream to be    worked up to give a stream rich in hydrogen and methane;-   (iii) at least one cooling unit for cooling the stream rich in    hydrogen and methane; and-   (iv) at least one cryogenic gas separation unit for separating the    stream rich in hydrogen and methane into at least one hydrogen-rich    stream and at least one methane-rich stream.

In addition, the object is achieved by a process for working up ahydrogen- and methane-comprising stream, which comprises the followingsteps:

-   (a) cooling of a stream to be worked up in at least one heat    exchanger;-   (b) purification of the stream to be worked up to give a stream rich    in hydrogen and methane in at least one separation unit;-   (c) cooling of the stream rich in hydrogen and methane in at least    one cooling unit;-   (d) separation of the stream rich in hydrogen and methane into at    least one hydrogen-rich stream and at least one methane-rich stream    in at least one cryogenic gas separation unit.

The invention makes it possible to recover hydrogen and methane in highpurity and reuse them in a simple way. This can be achieved using simplyand robustly constructed components which increase the life of theapparatus and minimize the outlay for monitoring and maintenance. Thus,in particular, the heat exchanger for precooling the stream to be workedup can be constructed simply and robustly since the requirements forefficiency of the heat exchanger are less demanding in the case of theapparatus of the invention and the process of the invention than in thecase of the plants known from the prior art. In addition, sensitivecomponents such as the cryogenic gas fractionation unit can adequatelybe protected against damage by separating off corrosive and freezablematerials from the stream. The removal of impurities thus leads torecovery of hydrogen and methane in high purity, which makes thesubsequent operation easier.

Furthermore, the invention provides for two-stage cooling of the streamto be worked up using the heat exchanger and the cooling unit in a firststage and the cryogenic gas separation unit in a second stage, whichincreases the stability of the work-up process.

This results, in particular, from the cooling unit being locatedupstream of the cryogenic gas separation unit and thus providing astream having an essentially constant temperature level before entryinto the cryogenic gas fractionation unit. For the present purposes, anessentially constant temperature level means temperature fluctuations inthe range +/−2° C.

In the following, values in % by volume are a measure of the proportionof a material in a mixture based on the total volume of the mixture. Forthe purposes of the invention, ppm based on the total volume of themixture is one part by volume per million. ppt based on the total volumeof the mixture refers to one part by volume per trillion. Standard m³/hrefers to standard cubic meters per hour, with the standard volume beinggiven for a pressure of 1 bar and a temperature of 20° C.

In an embodiment, the apparatus of the invention is used in the work-upof a stream taken from a dealkylation of alkyl-substituted aromatichydrocarbons. For this purpose, the apparatus of the invention can beinstalled downstream of a plant for dealkylation of alkyl-substitutedaromatic hydrocarbons. The individual components of the plant arepreferably arranged in the abovementioned order and, correspondingly,the steps of the process of the invention are likewise carried out inthe stated order.

In the plant for dealkylation of alkyl-substituted aromatichydrocarbons, nonaromatic hydrocarbons having six or more carbon atomsare, for example, aromatized in the presence of steam and a catalyst ina first step. In a second step, at least part of the product streamobtained, which comprises alkyl-substituted aromatic hydrocarbons, isreacted with the aid of hydrogen, optionally in the presence of acatalyst, to dealkylate the alkyl-substituted aromatic hydrocarbons.

After the dealkylation, the reaction product is rich in hydrogen,impurities and dealkylated aromatic hydrocarbons such as benzene ortoluene. The dealkylated aromatic hydrocarbons formed and ahydrogen-comprising gas phase can be separated off from the reactionproduct by conventional methods.

The reaction product of the dealkylation can be conveyed from thereactor into a heat exchanger and be cooled there, preferably to from 20to 100° C. It is advantageous to integrate the heat liberated here intothe process in order, for example, to heat the feed stream to thedealkylation or other streams to be heated, for example in a vaporizerof a column. A liquid phase and a hydrogen-comprising gas phase areformed in the heat exchanger.

The liquid phase formed, which comprises the dealkylated aromatichydrocarbon such as benzene and also excess water from the reactions, isfed to a phase separator and the organic phase is separated from theaqueous phase. The organic phase, which comprises the dealkylatedaromatic hydrocarbon, can optionally be purified further, for example bydistillation. The products obtained in the distillation can optionallybe recirculated to the reaction steps.

Before the hydrogen-comprising gas phase is fed to the apparatus of theinvention, it can be introduced into, for example, a separator or astripping column where high-boiling hydrocarbons, preferablyhydrocarbons having 4 and more carbon atoms and particularly preferablyhydrocarbons having 6 and more carbon atoms are separated off. This stepcan, for example, be carried out in a phase separator by adiabaticdepressurization. The bottom fraction obtained here, which compriseshigh-boiling hydrocarbons, in particular hydrocarbons having 6 and morecarbon atoms, can optionally be recirculated to the dealkylationprocess.

The overhead fraction obtained here, which comprises essentiallyhydrogen and methane, forms the stream to be worked up and is, exceptfor minor impurities, separated into at least one hydrogen-rich streamand at least one methane-rich stream in the apparatus of the invention.

The stream to be worked up preferably comprises at least 40% by volumeof hydrogen and at least 15% by volume of methane. The stream to beworked up more preferably comprises from 45 to 75% by volume,particularly preferably from 50 to 70% by volume, of hydrogen and from15 to 45% by volume, particularly preferably from 20 to 40% by volume,of methane. The stream to be worked up can additionally comprise up to10% by volume of impurities. Impurities which may be comprised are from1 to 5% by volume of ethane, from 0.2 to 2% by volume of nitrogen andfrom 0 to 7% by volume of hydrocarbons, for example from 0.3 to 4% byvolume of hydrocarbons having 6 and more carbon atoms, in particulararomatic hydrocarbons, and up to 2% by volume of C₁-C₅-hydrocarbons.Here, the individual components of the stream to be worked up areselected so that the sum of the volumes of the individual components isnot greater than 100%.

The stream to be worked up is fed to the apparatus of the invention at apressure of up to 100 bar, preferably up to 70 bar and particularlypreferably up to 60 bar. The volume flow of the stream which is fed inand is to be worked up is up to 50 000 standard m³/h, preferably up to40 000 standard m³/h and particularly preferably up to 35 000 standardm³/h. The temperature of the stream to be worked up can be up to 100° C.and is preferably in the range from 20 to 50° C. The stream to be workedup particularly preferably has a temperature in the range from 30 to 40°C.

The apparatus of the invention firstly provides a first cooling stagefor the work-up of the stream to be worked up. This cooling stagecomprises at least one heat exchanger which comprises at least one feedline for introduction of a stream to be worked up and to be cooled andat least one outlet for discharging a cooled stream to be worked up. Ina preferred embodiment, the heat withdrawn from the stream to be cooledand to be worked up is utilized in the work-up process by heating atleast one stream to be heated before exit from the apparatus. In thisway, worked up streams, in particular, can be heated before leaving theapparatus and thus be reused directly in process steps, for example fordealkylation or for heating.

The heat exchanger can be configured as a plate, helical orshell-and-tube heat exchanger. The stream to be cooled and worked up canbe conveyed in cocurrent, countercurrent, cross-cocurrent orcross-countercurrent relative to the stream to be heated. Here,cross-countercurrent refers to a mode of operation in which thematerials overall flow toward and past one another but cross at leastonce on their way. Analogously, cross-cocurrent refers to a mode ofoperation in which streams flowing in the same direction cross at leastonce. Preference is given to the countercurrent mode of operation.

The heat transfer power of the heat exchanger can be in the range from100 to 600 kW, preferably from 200 to 500 kW and particularly preferablyfrom 250 to 450 kW. The heat transfer power of the heat exchangerdepends critically on the heat transfer coefficient of the transfersurface provided, the volume flow of the streams and the desired averagetemperature difference between the cooled stream to be worked up and thestream to be heated.

The heat transfer coefficient is determined, inter alia, by the thermalconductivity of the material used. Here, a high thermal conductivity ofthe material used has a positive effect on the heat transfer power. Theheat exchanger or at least the surfaces participating in heat transfercan be made of metals, preferably steel such as an unalloyed orlow-alloy, ferritic steel, in particular stainless steel, copper,aluminum, and also glass, plastic, enamel, silicon carbide orcombinations thereof. In addition, an increased transfer area leads to abetter heat transfer power. For example, the plates of a plate heatexchanger or tubes of a shell-and-tube heat exchanger can have fins inorder to provide an increased heat transfer power in the smallestpossible volume.

In a preferred embodiment of the apparatus proposed according to theinvention, the heat exchanger is configured so that the averagetemperature difference between the cooled stream to be worked up and thestream to be heated is from 0.5 to 10° C., preferably from 1 to 9° C.,particularly preferably from 2 to 8° C. Thus, the stream to be worked upcan be cooled from an inlet temperature of, for example, 35° C. to anoutlet temperature of, for example, 5° C., while the at least one streamto be heated, in particular at least one worked up stream, can be heatedfrom, for example, 0° C. to 40° C.

To achieve average temperature differences in the abovementioned range,a heat exchanger having a moderate heat transfer power can besufficient. Preference is given to using a plate heat exchanger made ofstainless steel. As an alternative, it is also possible to use a heatexchanger made of, for example, aluminum which has a higher thermalconductivity than stainless steel. In this case, a smaller transfer areacompared to a stainless steel heat exchanger can be sufficient, althoughaluminum is more sensitive to corrosion because of, for example,ammonia- or sulfur-comprising residues in the stream to be worked up.Relatively large heat transfer areas can be achieved more cheaply in thecase of materials such as aluminum than in the case of stainless steel.

In a first step of the process of the invention, the stream to be workedup is introduced into a first cooling stage in order to cool it. Here,the stream to be worked up is firstly cooled by means of a heatexchanger of the above-described type. In an embodiment of the processproposed according to the invention, the stream to be worked up iscooled from an inlet temperature of not more than 100° C., preferably inthe range from 20 to 50° C. and particularly preferably in the rangefrom 30 to 40° C., to a temperature of not more than 15° C., preferablyin the range from −5 to 10° C. and particularly preferably in the rangefrom 0 to 7° C. In return, at least one stream to be heated, inparticular at least one worked up stream, can be heated from, forexample, 0° C. to 40° C.

In the apparatus proposed according to the invention, it is advantageousfor the heat exchanger to be followed by a separation unit in order toseparate off corrosive and high-boiling materials from the stream to beworked up. For example, ammonia-comprising materials, sulfur-comprisingmaterials, in particular hydrogen sulfides, or both can be comprised inthe ppm range in the stream to be worked up and can damage thesubsequent components, in particular the cryogenic gas separation unit.In addition, high-boiling materials such as water and hydrocarbonshaving 4 and more carbon atoms can freeze in the cryogenic gasseparation unit at temperatures below −100° C. and damage the cryogenicgas separation unit by formation of ice.

The separation unit can be made up of one part or a plurality of parts.In a preferred embodiment, the separation unit comprises at least onephase separator and/or at least one gas purification unit.

To separate off condensed components of the cooled stream to be workedup after passage through the heat exchanger, the heat exchanger can befollowed by at least one phase separator. In this, the condensedfraction of the gas to be worked up can, after cooling by means of theheat exchanger, be separated by, for example, adiabatic depressurizationfrom the gas phase of the stream to be worked up. In this way,high-boiling components, in particular high-boiling hydrocarbons,preferably hydrocarbons having 4 and more carbon atoms, particularlypreferably hydrocarbons having 5 and more carbon atoms and veryparticularly preferably hydrocarbons having 6 and more carbon atoms, andalso corrosive materials, in particular ammonia-comprising materials andsulfur-comprising materials, can be removed from the stream to be workedup.

To achieve improved removal of corrosive components in the phaseseparator, a scrubbing liquid which binds constituents of the stream tobe worked up can be introduced into the stream to be worked up before itenters the heat exchanger. The constituents of the stream to be workedup which go over can be solid, liquid and gaseous components. Asscrubbing liquid which takes up, in particular, corrosive components ofthe stream to be worked up can be a pure solvent such as water or steamor a suspension such as calcium hydroxide solution Ca(HO)₂.

In addition or as an alternative, a gas purification unit can beprovided within the separation unit. This serves to separate offhigh-boiling constituents, in particular high-boiling hydrocarbons,preferably hydrocarbons having 4 and more carbon atoms, for exampletoluene, and corrosive constituents such as water from the stream to beworked up. The gas purification unit can be configured as an adsorptivegas purification unit.

The adsorptive gas purification unit can be designed as a temperature-or pressure-swing adsorption. In contrast to temperature-swingadsorption, in which the regeneration of the adsorber (desorption) iscarried out by means of a temperature increase, the regeneration inpressure-swing adsorption takes place at a reduced pressure. To operatethe adsorptive separation process (pseudo)continuously, it is possibleto provide at least two adsorbers which are operated in parallel and ofwhich at any time at least one is in the adsorption phase and at leastone is in the regeneration phase.

In the apparatus proposed according to the invention, the gaspurification unit can be configured as an adsorptive gas purificationunit which is, in particular, designed for carrying out a continuoustemperature-swing adsorption. Examples of materials to be adsorbed inthe temperature-swing process are water, hydrogen sulfide and/orhydrocarbons, in particular hydrocarbons having 4 and more carbon atoms.Adsorbents can be molecular sieves, in particular zeolites, silica geland/or aluminum oxide, with different molecular sieves being able to becombined depending on the use.

In the process proposed according to the invention, the heat exchangerfor cooling the stream to be worked up is followed by a separation unitin order to obtain a stream rich in hydrogen and methane. For example,corrosive and/or high-boiling components can be separated off in theseparation unit from the stream to be worked up. The removal ofcorrosive components and/or high-boiling components can, as describedabove, be carried out in one step or a plurality of steps.

In a preferred embodiment of the process of the invention, it ispossible, as described above, to use at least one phase separator and/orat least one gas purification unit for separating off corrosivecomponents and/or high-boiling components. In the phase separator,high-boiling constituents, in particular high-boiling hydrocarbons,preferably hydrocarbons having 4 and more carbon atoms, particularlypreferably hydrocarbons having 5 and more carbon atoms and veryparticularly preferably hydrocarbons having 6 and more carbon atoms,can, for example, be removed from the stream to be worked up. Inaddition, corrosive components in the stream to be worked up, e.g.ammonia-comprising materials and sulfur-comprising materials, can bebound by introduction of a scrubbing liquid before entry into the heatexchanger. The corrosive components of the cooled stream to be worked upcan, after the stream has passed through the heat exchanger, beseparated off in a phase separator. To effect further purification ofthe stream to be worked up, it is possible to use a continuouslyoperated temperature-swing adsorption in order, for example, to separatewater vapor from the stream to be worked up. For example, water,hydrogen sulfide and/or hydrocarbons, in particular hydrocarbons having4 and more carbon atoms, are removed adsorptively in the adsorptionoperated by the temperature-swing method using molecular sieves, inparticular zeolites, silica gel and/or aluminum oxide as adsorbents.

The stream rich in hydrogen and methane which is extracted from theseparation unit is preferably essentially free of corrosive componentsand/or high-boiling components such as high-boiling hydrocarbons, inparticular hydrocarbons having 4 and more carbon atoms,ammonia-comprising materials, sulfur-comprising materials and water,with the stream rich in hydrogen and methane comprising only traces of afew ppm of corrosive and freezable impurities.

In an embodiment of the apparatus proposed according to the invention,the cooling unit comprises at least one refrigeration unit. In therefrigeration unit, indirect cooling of the stream to be worked upoccurs, with the refrigeration unit taking up heat at below ambienttemperature and giving it off at a higher temperature. Compressionand/or sorption refrigeration machines to which the required energy issupplied entirely as mechanical work or in the form of heat aretypically used for this purpose. Electrically operated refrigerationunits are also conceivable.

The cooling unit can have a cooling power of from 10 to 500 kW,preferably from 50 to 300 kW, particularly preferably from 80 to 200 kW.By means of these powers, the hydrogen- and methane-rich stream whichhas been precooled in the heat exchanger and after passing through theseparation unit can have a temperature in the range from 0 to 15° C. canbe cooled to a constant temperature level in the range from −10 to 5° C.Here, the constant temperature level comprises small deviations of lessthan 2° C.

In a particularly preferred embodiment of the apparatus of theinvention, the cooling unit is located directly upstream of thecryogenic gas separation unit. This is particularly advantageous sincethe first cooling stage ensures an essentially constant temperaturebefore entry into the cryogenic gas separation unit and thus stabilizesthe process of cryogenic gas fractionation. Furthermore, a simplyconstructed cooling unit having moderate power is sufficient to achievestabilization of the process.

In an embodiment, the refrigeration unit comprises a compressionrefrigeration machine which is equipped with at least one compressionelement, at least one expansion element and at least two heatexchangers. Here, the compression element can be realized by means of amechanical compressor, for example a conventional compressor. Theexpansion element can be configured as throttle device, for example asan expansion valve. For the operating circuit, a compression element andan expansion element and two heat exchangers can be connected in acircuit so that the heat exchangers are located on both sides betweencompression element and expansion element. As refrigerant for thethermodynamic cycle, it is possible to use, for example, carbon dioxide(CO₂) or ammonia (NH₃).

In an embodiment of the process of the invention, a cooling unit of theabove-described type is used. The stream rich in hydrogen and methanecan be cooled by means of the cooling unit having a cooling power in therange from 10 to 200 kW to an essentially constant temperature level.For the present purposes, an essentially constant temperature level is atemperature level of the stream which is constant to within smalldeviations of less than 2° C. The cooling by means of the cooling unitis preferably carried out directly before the separation of the streamrich in hydrogen and methane in the cryogenic gas separation unit. Inthis way, the hydrogen- and methane-rich stream which has been precooledby the heat exchanger can be cooled by up to 30° C., preferably by from5 to 10° C., with a simply constructed cooling unit having a moderatepower being able to be used. In addition, the first cooling stageensures an essentially constant temperature level before entry into thecryogenic gas separation unit, which stabilizes the work-up process, inparticular the cryogenic gas separation.

Preference is given to using a compression refrigeration unit forcooling the stream rich in hydrogen and methane. As refrigerant in thecompression refrigeration unit, it is possible to use, for example,carbon dioxide (CO₂) or ammonia (NH₃).

The separation into at least one hydrogen-rich stream and at least onemethane-rich stream in the present invention takes place in thecryogenic gas separation unit. The synthesis gas is typically cooled, bymeans of expansion units and by indirect heat exchange with streams tobe heated, to such an extent that partial condensation occurs to form atleast one methane-rich liquid fraction and at least one hydrogen-richgas fraction which are subsequently separated in a phase separator. Themethane-rich liquid fraction can be revaporized and heated and taken ingaseous form from the cryogenic gas separation unit.

In an embodiment, the cryogenic gas separation unit comprises a cascadeof heat exchangers and/or expansion units which are enclosed by athermally insulated housing. The cascade of heat exchangers and/orexpansion units necessary for carrying out the condensation process isusually arranged in a steel housing and insulated with perlite in orderto minimize input of heat. Furthermore, to prevent synthesis gascomponents leaking from the cryogenic gas separation unit fromaccumulating within the housing and to prevent ice formation due tointrusion of surrounding air, the interior of the housing can becontinuously flushed with a stream of nitrogen at a somewhat elevatedpressure.

The cryogenic gas separation unit is usually operated under cryogenicconditions and exploits the Joule Thomson effect and low-temperaturecooling in order to separate hydrogen from methane. In this way, thestream rich in hydrogen and methane can be cooled in the cryogenic gasseparation unit to very low temperatures of less than −100° C., withcooling down to −180° C., preferably in the range from −150 to −165° C.,being able to be achieved. At these temperatures, methane condenses fromthe gaseous stream and the methane fraction can be taken off as a liquidfraction in a phase separator.

In a preferred embodiment, a plurality of expansion stages, preferablytwo expansions, are provided in the cryogenic gas separation unit.Methane-rich streams having different pressures can be formed as aresult. Thus, the methane-rich liquid fraction formed in the firstexpansion can be discharged and subsequently be revaporized, warmed anddischarged in gaseous form from the gas separation unit. Thismethane-rich stream can have a pressure in the range from 5 to 10 bar.In a second expansion stage, the stream rich in hydrogen and methane canbe cooled to a lower temperature than in the first expansion stage. Thisgives a second methane-rich stream which has a pressure in the rangefrom 0 to 4 bar. Finally, the gas fractions from the two expansionstages are combined and form the hydrogen-rich stream.

Overall, at least one hydrogen-rich stream and at least one methane-richstream are obtained at the outlet of the cryogenic gas separation unit.In a preferred embodiment, these streams are heated in the heatexchanger of the first cooling stage by means of the stream to be workedup. In this way, the worked up hydrogen-rich stream and the worked upmethane-rich stream can be heated before recirculation to processes suchas the dealkylation.

The methane-rich stream comprises at least 70% by volume, preferably inthe range from 75 to 95% by volume, of methane. In addition, up to notmore than 10% by volume, preferably up to not more than 5% by volume, ofhydrogen can be comprised in the methane-rich stream. Furtherimpurities, in particular ethane, ethene, propene, propane, nitrogen,oxygen and carbon monoxide can be comprised in a proportion of not morethan 20% by volume, preferably not more than 15% by volume, in themethane-rich stream.

In an embodiment of the proposal according to the invention, a pluralityof methane-rich streams which can have different pressures in the rangefrom 0 to 20 bar and in each case come from one of the plurality ofexpansion stages of the cryogenic gas separation unit are obtained.Thus, for example, a methane-rich stream having a pressure of from 0 to4 bar (low-pressure stream) and a methane-rich stream having a pressureof from 5 to 10 bar (high-pressure stream) can be produced. In such acombination, the composition of the high-pressure stream and of thelow-pressure stream can differ in that the low-pressure stream isobtained in the cryogenic gas separation unit from a condensate having arelatively low temperature. Thus, the low-pressure stream can haveproportions of gases, for example oxygen, which condense in atemperature range which is, for example, achieved only after the secondexpansion.

The work-up according to the invention makes it possible to obtain ahydrogen-rich stream having a purity of at least 85% by volume,preferably in the range from 90 to 95% by volume, from the stream to beworked up. In addition, up to 10% by volume, preferably up to 8% byvolume, of methane can be comprised in the hydrogen-rich stream. Furtherimpurities, in particular nitrogen and carbon monoxide, can be comprisedin a proportion of not more than 5% by volume, preferably not more than2% by volume, in the hydrogen-rich stream.

After leaving the cryogenic gas separation unit, the hydrogen-richstream can have an exit pressure corresponding to the inlet pressureupstream of the cryogenic gas separation unit. The pressure canconsequently be up to 100 bar, preferably up to 70 bar and particularlypreferably up to 60 bar.

The at least one methane-rich stream can be utilized as heating gas, forexample for firing a steam cracking furnace. The joule value of the atleast one methane-rich stream can be from 35 000 to 50 000 kJ/m³,preferably from 40 000 to 45 000 kJ/m³. The joule value is a measure ofthe specifically useable quantity of heat obtainable from a fuel,without taking into account any heat of condensation. The at least onehydrogen-rich stream typically has a lower joule value of from 10 000 to20 000 kJ/m³ and is preferably reused in the hydrodealkylation ofalkyl-substituted hydrocarbons.

EXAMPLES

An illustrative embodiment of the invention is shown as a process flowdiagram in FIG. 1. The process of the invention and the associatedapparatus 100 for working up a stream of material are, in the presentexample, described in the form of a product stream 1.1 from ahydrodealkylation plant. After the dealkylation, the reaction product istypically rich in dealkylated aromatic hydrocarbons, hydrogen andmethane.

The aromatic hydrocarbons and a recycle gas to be worked up aretherefore firstly separated off from the reaction product of thehydrodealkylation. After a prepurification stage in which high boilershaving a boiling point of more than 50° C. are separated off, therecycle gas 1.1 to be worked up can, with addition of water 2, firstlybe introduced into the apparatus 100 according to the invention. Theaddition of water makes it possible to bind corrosive compounds such asammonia and sulfur-comprising materials in water and, after goingthrough a heat exchanger KS1, separate them off in a restrictor A1.

A volume flow of from about 30 000 to 35 000 standard m³/h of recyclegas 1.2 is fed at a pressure of from about 50 to 60 bar and atemperature of from 30 to 40° C. to the apparatus 100 according to theinvention. An illustrative composition of the recycle gas 1.2 beforeentry into the apparatus 100 is shown in Table 1. This comprisesessentially hydrogen and methane together with relatively lowconcentrations of, for example, ethane, ethene, propane, C₄- andC₆-hydrocarbons (in Table 1, abbreviated as C₄—HC and C₆—HC,respectively), oxygen, nitrogen and carbon monoxide as impurities.

TABLE 1 Recycle gas (% by volume) i- n- 1,3- Hydrogen Methane EthaneEthene Propane Propene Butane Butane Butadiene Min. 53.70 31.01 1.610.01 0.03 0.00 0.00 0.00 0.01 value Average 60.43 35.20 2.99 0.03 0.050.00 0.00 0.00 0.01 value Maxim. 66.89 40.84 4.68 0.04 0.09 0.00 0.000.00 0.01 value Carbon Gas Lower Upper joule C₄HC C₆HC Oxygen Nitrogenmonoxide density joule value value Min. value 0.00 0.42 0.34 0.34 0.090.3132 19435 22068 Average 0.00 0.91 0.40 0.64 0.17 0.3789 21954 24774value Maxim. 0.00 3.47 0.46 1.83 0.20 0.5141 27403 30620 value

The first cooling stage of the apparatus 100 according to the inventioncomprises a stainless steel plate heat exchanger KS1 having a transferpower of at least 300 kW, in order to cool the recycle gas 1.2 to atemperature in the range from 3 to 7° C. In a subsequent restrictor A1,components of the recycle gas 2 condensed by cooling are separated off.Here, it is first and foremost corrosive components and high-boilinghydrocarbons such as hydrocarbons having 4 and more carbon atoms,preferably 5 and more carbon atoms and particularly preferably 6 andmore carbon atoms, which are condensed and can optionally berecirculated to the dealkylation process.

Within the first cooling stage of the apparatus 100 according to theinvention, further purification steps A which filter high-boilingcomponents such as water from the recycle gas 4 can be carried out. Thisis of particular interest in respect of the cryogenic gas separationunit KS3 used for the low-temperature fractionation and the componentsof the cryogenic gas separation unit in order to increase the life andreduce the need for maintenance. The recycle gas 4 is dried in anadsorption unit A2, A2′ in which water is adsorbed using molecularsieves. In addition, high-boiling hydrocarbons, in particularhydrocarbons having 4 and more carbon atoms, are adsorbed in theadsorption unit A2, A2′ using further adsorbents known to those skilledin the art. The regeneration of the adsorbents is effected by desorptionby means of hydrogen at elevated temperature. To achieve a continuousprocess in the apparatus 100, two adsorption units A2 and A2′ aretherefore operated in parallel so that a functional adsorption unit A2,A2′ is available during the regeneration phase. After going through theindividual separation stages A, which can also be configured differentlyfrom the embodiment shown in FIG. 1, the recycle gas 5 comprisesessentially hydrogen and methane with less than 10% by volume of otherimpurities.

As last step of the first cooling stage, the recycle gas 5 is cooled bymeans of a refrigeration unit KS2 to a temperature level in the regionof 0° C. and thus fixes the temperature of the recycle gas 6 at theinlet to the cryogenic gas separation unit KS3. This makes it possibleto ensure that the temperature of the recycle gas 6 at the inlet intothe cryogenic gas separation unit KS3 is essentially constant and anytemperature fluctuations due, for example, to the temperature change inthe regeneration in the adsorption unit A2 or A2′ can be prevented.

In the cryogenic gas separation unit KS3, which represents the secondcooling stage of the work-up process of the invention, thelow-temperature fractionation of the recycle gas 6, which comprisesessentially hydrogen and methane, is carried out. The recycle gas afterprepurification by means of the units A typically comprises somewhatmore than 60% by volume of hydrogen and somewhat more than 35% by volumeof methane. In the cryogenic gas separation unit KS3, the recycle gas 6is cooled by means of cascades of heat exchangers and expansion stagesto very low temperatures in the range from −150 to −165° C., with thedew point of methane being attained at these low temperatures. The verylow temperatures in the cryogenic gas separation unit KS3 are typicallyreached in two expansions. Details of the cryogenic gas separation unitare not shown in FIG. 1. In a first expansion to very low temperatures,a first liquid fraction of methane is separated off from the recycle gas6. This first liquid fraction is, before leaving the cryogenic gasseparation unit KS3, preferably reheated in heat exchangers againststreams to be cooled and fed in gaseous form under a pressure in therange from 5 to 10 bar back into the heat exchanger KS1 where furtherheating against the feed stream 1.2 to be cooled takes place.

TABLE 2.1 Methane-rich stream (high-pressure) (% by volume) n- 1,3-Hydrogen Methane Ethane Ethene Propane Propene i-Butane Butane ButadieneMin. 4.14 80.10 5.38 0.04 0.03 0.01 0.01 0.00 0.01 value Average 4.6185.53 8.94 0.08 0.15 0.01 0.01 0.00 0.01 value Maxim. 4.85 88.70 14.680.13 0.22 0.01 0.01 0.00 0.01 value Carbon Gas Lower Upper joule C₄HCC₆HC Oxygen Nitrogen monoxide density joule value value Min. value 0.000.00 0.00 0.24 0.13 0.7543 37197 41174 Average 0.00 0.00 0.00 0.49 0.190.7724 38091 42140 value Maxim. 0.00 0.00 0.00 0.70 0.23 0.8013 3948943644 value

The gaseous fraction formed after the first expansion is cooled furtherin a second expansion in order to obtain even lower temperatures andseparate off further amounts of methane from the gaseous fraction. Afterthe second expansion, a second liquid fraction of essentially methane isagain formed. The second liquid fraction is also warmed again by meansof streams to be cooled in the heat exchanger cascades of the cryogenicgas separation unit KS3 before leaving the cryogenic gas separation unitKS3. The gaseous fraction formed after the second expansion comprisesessentially hydrogen and thus forms the stream of reusable pure gas 7which is likewise warmed by means of streams to be cooled before leavingthe cryogenic gas separation unit KS3.

After the low-temperature fractionation, a total of three worked upstreams thus leave the cryogenic gas separation unit KS3: twomethane-rich streams 9 and 8 at differing pressures and a hydrogen-richstream 7. These are present in the gaseous phase and all have atemperature in the region of the inlet temperature of about 0° C. as hasbeen achieved by means of the refrigeration unit KS2 before entry intothe cryogenic gas separation unit KS3.

The worked up methane-rich stream 9 resulting from the first expansionis typically under a pressure in the range from 5 to 10 bar. The workedup methane-rich stream 8, on the other hand, comes from the secondexpansion and therefore has a lower pressure in the range from 0 to 4bar. Illustrative compositions of the streams 8 and 9 are shown inTables 2.1 and 2.2.

TABLE 2.2 Methane-rich stream (low-pressure) (% by volume) i- n- 1,3-Hydrogen Methane Ethane Ethene Propane Propene Butane Butane ButadieneMin. 4.14 81.02 4.94 0.02 0.02 0.01 0.01 0.00 0.00 value Average 4.6585.69 8.72 0.08 0.15 0.01 0.01 0.00 0.00 value Maxim. 4.95 89.32 13.960.12 0.23 0.01 0.01 0.00 0.00 value Carbon Gas Lower joule Upper jouleC₄HC C₆HC Oxygen Nitrogen monoxide density value value Min. value 0.000.00 0.12 0.35 0.14 0.7458 36883 40845 Average 0.00 0.00 0.16 0.53 0.190.7718 38044 42087 value Maxim. 0.00 0.00 0.19 1.03 0.22 0.7970 3928443426 value

The worked up methane-rich streams 8, 9 are, before further use,recirculated as heating gas to the heat exchanger KS1 and heated againstthe stream 1.2 to be cooled. The heated streams 12 and 13 aresubsequently combined to form a total stream 15, with the pressureconditions of the stream 13 being matched to those of the stream 12.

Table 3 lists an illustrative composition of the hydrogen-rich stream 7which forms the reusable pure gas 7. This stream has a temperature and apressure which correspond to the conditions of the hydrogen-rich stream6 before entry into the cryogenic gas separation unit KS3. Thishydrogen-rich stream 6 typically has a pressure in the range from 40 to80 bar.

TABLE 3 Pure gas (% by volume) 2-Me- Propane (i- n- 1,3- HydrogenMethane Ethane Ethene Propane Propene Butane) Butane Butadiene Min.90.64 6.33 0.00 0.00 0.00 0.00 0.00 0.00 0.00 value Average 91.68 7.680.00 0.00 0.00 0.00 0.00 0.00 0.00 value Maxim. 93.47 8.74 0.00 0.000.00 0.00 0.00 0.00 0.00 value Lower Carbon Gas joule Upper joule C₄HCC₆HC Oxygen Nitrogen monoxide density value value Min. 0.00 0.00 0.000.06 0.01 0.1318 12344 14424 value Average 0.00 0.00 0.00 0.50 0.140.1454 12657 14763 value Maxim. 0.00 0.00 0.00 0.73 0.18 1.1525 1292915057 value

After separation of the recycle gas 1.1 into two methane-rich streamsand a hydrogen-rich stream, the streams 7, 8, 9 are recirculated to theheat exchanger and heated against the stream 1.2 to be cooled. Theheated pure gas 11 is subsequently introduced in the total stream 14into the plant for dealkylation of alkyl-substituted aromatichydrocarbons.

Furthermore, part of the reusable pure gas 7 in the embodiment shown inFIG. 1 is branched off before heating in the heat exchanger KS1 and isused for regeneration of the adsorption unit A2 or A2′. To regeneratethe adsorption unit A2 or A2′, the substream 10.1 of the pure gas 7 isfed into a heater A3, for example an electric heater, and heated to atemperature of from 150 to 350° C., preferably from 200 to 260° C. Inother embodiments, the substream of the reusable pure gas can also bebranched off from stream 11 after heating in the heat exchanger KS1 andbe fed to a heater.

The stream 10.2 is subsequently fed into the adsorption unit A2 or A2′to be regenerated. In addition, it can be provided for part of thestream 10.2 to be added to the total stream of the heating gas 15. Afterregeneration, the stream 10.3 is added to the heated stream of reusablepure gas 11 and the total stream 14 is fed to the plant for dealkylationof alkyl-substituted aromatic hydrocarbons.

To be able to reuse the three worked up streams 7, 8, 9 as directly aspossible after the apparatus 100, these are in each case heated in theheat exchanger KS1 of the first cooling stage against the stream 1.2 tobe cooled and the methane-rich streams 12, 13 and the hydrogen-richstream 11 leave the apparatus at temperatures of more than 20° C. Inthis way, the apparatus 100 according to the invention can be operatedefficiently in the recycle mode, with virtually all resources beingreused. In particular, the hydrogen-rich stream 11 can be recirculatedto the hydrodealkylation (not shown). The two methane-rich streams 12and 13 can be used as heating gas, for example for firing a steamcracking furnace (not shown).

LIST OF REFERENCE NUMERALS

-   1.1, 1.2 stream to be worked up-   2 water-   3 stream after heat exchanger-   4 stream after restrictor-   5 stream after separation stage-   6 stream after cooling unit-   7 hydrogen-rich pure gas after cryogenic gas separation unit-   8 methane-rich heating gas (low-pressure)-   9 methane-rich heating gas (high-pressure)-   10.1 regeneration stream of pure gas-   10.2 heated regeneration stream of pure gas-   10.3 recycle stream of pure gas after regeneration-   11 hydrogen-rich pure gas after heat exchanger-   12 methane-rich heating gas (low-pressure) after heat exchanger-   13 methane-rich heating gas (high-pressure) after heat exchanger-   14 total stream of pure gas-   15 total stream of heating gas-   100 apparatus for working up a stream of material-   KS1 heat exchanger-   KS2 refrigeration unit-   KS3 cryogenic gas separation unit-   A separation stage-   A1 phase separator-   A2, A2′ adsorptive separation unit-   A3 heater

1. An apparatus (100) for working up a hydrogen- and methane-comprisingstream (1.1), which comprises the following components: (i) at least oneheat exchanger (KS1) for cooling a stream (1.1) to be worked up; (ii) atleast one separation unit (A, A1, A2, A2′) for purifying the stream (3)to be worked up to give a stream (5) rich in hydrogen and methane; (iii)at least one cooling unit (KS2) for cooling the stream (5) rich inhydrogen and methane; and (iv) at least one cryogenic gas separationunit (KS3) for separating the stream (6) rich in hydrogen and methaneinto at least one hydrogen-rich stream (7) and at least one methane-richstream (8, 9).
 2. The apparatus (100) according to claim 1, wherein thestream (1.1, 1.2, 3) to be worked up comprises at least 40% by volume ofhydrogen and at least 15% by volume of methane.
 3. The apparatus (100)according to claim 1, wherein the at least one heat exchanger (KS1) isconfigured as a plate, helical or shell-and-tube heat exchanger.
 4. Theapparatus (100) according to claim 1, wherein the heat exchanger (KS1)is made of steel, copper, aluminum, glass, plastic, enamel and/orsilicon carbide.
 5. The apparatus (100) according to claim 1, whereinthe separation unit (A, A1, A2, A2′) comprises at least one phaseseparator (A1) and/or at least one gas purification unit (A2, A2′). 6.The apparatus (100) according to claim 5, wherein the gas purificationunit (A2, A2′) is configured as an adsorptive gas purification unit. 7.The apparatus (100) according to claim 5, wherein the gas purificationunit (A2, A2′) is configured as a continuously operatedtemperature-swing adsorption.
 8. The apparatus (100) according to claim1, wherein the cooling unit (KS2) is located directly upstream of thecryogenic gas separation unit (KS3).
 9. A process for working up ahydrogen- and methane-comprising stream (1.1, 1.2), which comprises thefollowing steps: (a) cooling of a stream (1.1, 1.2) to be worked up inat least one heat exchanger (KS1); (b) purification of the stream (3) tobe worked up to give a stream (5) rich in hydrogen and methane in atleast one separation unit (A, A1, A2, A2′); (c) cooling of the stream(5) rich in hydrogen and methane in at least one cooling unit (KS2); (d)separation of the stream (6) rich in hydrogen and methane into at leastone hydrogen-rich stream (7) and at least one methane-rich stream (8, 9)in at least one cryogenic gas separation unit (KS3).
 10. The processaccording to claim 9, wherein the stream (1.1, 1.2) to be worked up iscooled by means of the at least one heat exchanger (KS1) from an inlettemperature of not more than 100° C. to a temperature of not more than15° C.
 11. The process according to claim 9, wherein corrosive and/orhigh-boiling components are separated off in the separation unit (A, A1,A2, A2′) from the stream (1.1, 1.2, 3) to be worked up.
 12. The processaccording to claim 9, wherein the cooling unit (KS2) cools the stream(5) rich in hydrogen and methane to an essentially constant temperaturelevel.
 13. The process according to claim 9, wherein the cooling bymeans of the cooling unit (KS2) is carried out directly before theseparation of the stream (6) rich in hydrogen and methane in thecryogenic gas separation unit (KS3).
 14. The process according to claim9, wherein the stream (6) rich in hydrogen and methane is cooled to atemperature of less than −100° C. in the cryogenic gas separation unit.15. The process according to claim 9, wherein the hydrogen-rich stream(7) is reused in a process for the dealkylation of alkyl-substitutedaromatic hydrocarbons.
 16. The process according to claim 9, wherein themethane-rich stream (8, 9) is utilized as heating gas.
 17. The processaccording to claim 9, wherein the hydrogen- and methane-comprisingstream (1.1, 1.2) is taken from a process for the dealkylation ofalkyl-substituted aromatic hydrocarbons.